Recent rapid development of mobile communication systems and mobile communication terminals has caused drastic increase of data amount requested by users. This demands more bandwidth from limited frequency resources for mobile communication systems, which has been addressed, but not in a fully satisfactory manner, by an emerging technology that utilizes millimeter waves having wavelengths in the order of millimeters. The next-generation 5G system which has been discussed recently actually plans to employ small cell backhaul systems utilizing millimeter waves with frequency of, for example, 28 GHz or 60 GHz.
Processing the millimeter waves requires waveguide filters which have been mainly used in technical fields such as defense and satellite communications. Furthermore, in mobile communication systems, a waveguide filter of a cavity type is used so as to be able to satisfy the requirements for high frequency bands and high performance filtering characteristics.
A waveguide filter utilizes a resonance phenomenon caused by its physical structure, in which a tubular waveguide is designed to have a length corresponding to the frequency filtering characteristics thereof. For example, a waveguide filter may be classified into a cavity type which makes use of metal blocks, and a type having a waveguide with a dielectric resonance element, such as ceramic therein. In case of high frequency bands such as millimeter waves, a cavity type waveguide filter has less dielectric loss, and thus is more suitable.
FIG. 1 is an exploded perspective view showing an example of a typical cavity type waveguide filter (main part). Referring to FIG. 1, a cavity type waveguide filter generally includes a first case 10 (e.g., a housing) and a second case 11 (e.g., a cover) as basic components. A plurality of partitions 131, 132, 133, 134, 135, 136, 137, 138 is also provided for implementing the interior of the waveguide based on the relevant filtering frequency.
A cavity type waveguide filter is usually composed of rectangular parallelepiped resonance stages which generate resonance at a desired frequency, and two partitions (also known as, “Iris”) installed facing each other for establishing a coupling between the resonance stages. In the example of FIG. 1, the plurality of partitions 131 to 138 forms first to third resonance sections 121, 122 and 123 to be connected in a row in the waveguide. Here, the first resonance section 121 is preceded by a formation of input section 112, and the third resonance section 123 has a trailing output section 114, to provide incoming and outgoing power feeders, respectively. In addition, by appropriately designing the mutual spacing of every two partitions formed facing each other between the respectively resonance sections and the input and output sections, the amount of signal coupling between the sections is appropriately set.
In the above-described waveguide filter, the cross-sectional shape of the waveguide is rectangular (square or rectangular) in some typical cases, where the transverse length (a) and vertical length (b) of the internal cross section of the waveguide influence the cutoff frequency characteristics of the relevant filter, and these lengths can be designed to have virtually normalized numerical values according to the relevant filtering frequency. In addition, according to the wavelength λ of the relevant filtering frequency, the waveguide lengths of the first to third resonance sections 121, 122 and 123 along with the input and output sections 112, 114 are appropriately set, so that their waveguide lengths have the values of λ/2, λ/4, λ/8, and so on.
The cavity type waveguide filter as shown in FIG. 1 has a structure in which, for example, the first to third resonance sections 121, 122 and 123 are connected to each other in a row, that is a 3-stage filter structure on one surface. It will be understood by those skilled in the art that the filter may be designed with four or more stages or one or two stages depending on the number of resonance sections connected to each other in a row.
An example of such a cavity type waveguide filter is disclosed in U.S. Patent Publication No. 2003/0206082 (entitled “WAVEGUIDE FILTER WITH REDUCED HARMONICS,” inventors: “Ming Hui Chen,” “Wei-Tse Cheng,” Publication Date: Nov. 6, 2003).
Meanwhile, the first case 10 and the second case 11, as shown in FIG. 1, which constitute the waveguide in the cavity type waveguide filter, may be manufactured by a cutting processing for higher machining precision. At this time, the first case 10 and the plurality of partitions 131-138 are integrally formed from a single base material by machining. The first case 10 and the second case 11 may then be joined together by screw fastening or welding.
In order to compensate for machining tolerance in a waveguide filter having such a structure, it is common to employ a structure in which a frequency tuning screw or bar is inserted into a resonance section of the resonance structure at an appropriate place via, for example, a screw hole or the like formed in the second case 11. Likewise, adjacent paired partitions installed between the resonance sections may be structured to have tuning screws and bars for tuning the coupling between the resonance sections by inserting them into a screw hole or the like formed in the second case 11.
When implementing a waveguide filter for processing millimeter waves, the very short length of the frequency wavelength for processing requires the overall resonance structure to be sized to be very small and adjacent paired partitions installed between the resonance sections to be very closely spaced. This makes it difficult in practice to employ a structure that installs the aforementioned frequency tuning screws and the coupling tuning screws. For example, the distance between two partitions that are installed in pairs between the resonance sections may be less than 1 mm which is too small to actually place a tuning screw therein.
Thus, the difficulty to employ a structure that involves a tuning screw installation when implementing a waveguide filter for processing millimeter waves compels manufacturers to become reliant on a highly precise manufacturing process to have such machining tolerance that does not require a tuning process. In other words, implementing a waveguide filter for processing millimeter waves requires extremely high processing accuracy in order to realize the designed structure into an actual product. For example, the machining tolerance of about 0.01 mm or less may be required at the interval between adjacent paired partitions facing each other.
However, the requirement of very precise machining tolerances aggravates the difficulty of machining work and lengthens the machining time, which results in an increase in machining costs, decreased production yield to render mass production difficult. For the purpose of reducing the processing cost, it is the current practice to reduce the performance of a corresponding filter, or to select and use a filter product that satisfies the required performance after producing a plurality of filters (that is, a product which does not satisfy the required performance is treated as a defective one). Due to these reasons, the market price of high performance waveguide filters remains very high.